This invention relates to a switching circuit which handles a high frequency such as a microwave, and more particularly to a switching circuit which is suitably used where a high isolation is required or where a matching state is required upon switching off.
In radio communication in a cellular system or the like, the signal level is varied every moment by a variation of the distance to a base station, by an influence of fading which occurs in a transmission line or the like. In a terminal, it is a common practice to adjust a received signal which exhibits such a level variation to a fixed signal level by absorbing a variation of the level by a gain adjustment element, such as an amplifier or an attenuator, with which gain adjustment can be performed and then transmit the signal of the adjusted level to a demodulator. On the other hand, in transmission, in order to supply a fixed signal level to a base station, it still is a common practice to adjust a signal to a desired level by a gain adjustment element and then transmit the signal of the adjusted level as a signal output of the terminal.
From such demands as described above, generally a terminal is essentially required to have some gain adjustment element for both transmission and reception. However, the required adjustment width in gain is different among different systems. If the variation width in gain required by a system extends over 80 dB to 90 dB, then from the limitations in isolation, dynamic range and so forth of a device, it is very difficult to realize the variation width of a signal by means of a single gain adjustment element when the possibility in realization or the cost is taken into consideration.
Therefore, it is common to realize such a variation width of a signal by dividing a gain adjustment element into a plurality of elements. Referring to FIG. 1, there is shown a transmission system for radio communication such as a cellular system in which such common measures are employed.
If it is assumed that the system requires gain adjustment of a transmission signal by 80 dB, then a level variation of a signal by 80 dB must be exhibited at an input terminal 9a of a transmission antenna 9. Therefore, the level variation of a signal by 80 dB is distributed such that, for example, gain adjustment by 50 dB is performed by an intermediate frequency (IF) stage and gain adjustment by the remaining 30 dB is performed by a high frequency (radio frequency; RF) stage.
More particularly, an IF signal of a fixed level inputted from a terminal 1 is supplied through an IF signal line 2 to a first variable gain amplifier 3, by which gain adjustment is performed in the width of 50 dB. Then, the IF signal from the first variable gain amplifier 3 is mixed with a local oscillation frequency by a frequency mixer 4 so that it is frequency converted into a RF signal, and then, unnecessary frequency components, such as an image signal or the like, are removed from the RF signal by a band-pass filter 5.
The RF having passed through the band-pass filter 5 undergoes gain adjustment in the width of 30 dB by a second variable gain amplifier 6 and then undergoes fixed signal amplification by a power amplifier 7, after which whereafter it is supplied with a desired output power to the transmission antenna 9 through a band-pass filter 8. In this instance, the level difference of the variation of the transmission signal of the devices from the input terminal 1 of the transmission system to the input terminal 9a of the transmission antenna 9 may be 50 dB in the maximum.
By dividing the gain adjustment element into the first variable gain amplifier 3 and the second variable gain amplifier 6 and compressing the difference between the maximum level and the minimum level of a signal at each of nodes from the input terminal 1 of the transmission system to the input terminal 9a of the transmission antenna 9 in this manner, the dynamic range of each element can be minimized.
Meanwhile, a transmission system of a construction wherein a pass-band of a band-pass filter is divided into a plurality of subbands is known in the art. A transmission system of the type just described is provided because, when each of the band-pass filters 5 and 8 of FIG. 1 is to be implemented, depending upon a frequency characteristic required for a filter of a system, if it is intended to implement the filter with a single device, the filter requires a physically very large volume, resulting an electrically high loss in the pass band.
In particular, in order to eliminate such a problem as just described, the transmission system of FIG. 1 is modified such that, as seen in FIG. 2, dividing the pass-band of each of the band-pass filters 5 and 8 into two subbands, band-pass filters 5a, 5b and 8a, 8b having different pass bands are provided in place of the band-pass filters 5 and 8, respectively, and single pole double throw (SPDT) switches 10, 11 and 12, 13 are provided on the input and output sides of the band-pass filters 5a, 5b and 8a, 8b, respectively, so that one of two routes may be selected in each stage in accordance with a frequency of a signal. In this instance, even if the volumes or losses of the SPDT switches 10, 11 and 12, 13 are discounted, it is sometimes possible to realize a filter of a small volume and a low loss, and this is advantageous in space savings and power savings as an entire system.
However, in the transmission system described above with reference to FIG. 1 wherein gain adjustment is performed by the two variable gain amplifiers 3 and 6, power must always be supplied to the two variable gain amplifiers 3 and 6. Particularly with the power amplifier 7, since the power load efficiency at a low signal input level generally exhibits a remarkable drop, even if gain adjustment is performed by the variable gain amplifier 6, a further loss in power consumption is invited. This causes a serious problem in a radio communication system such as a cellular system from the point of view of assurance of a service time of a terminal.
Further, with the transmission system described above with reference to FIG. 2, since the SPDT switches 10, 11 and 12, 13 must be provided to switch the band-pass filters 5a, 5b and 8a, 8b, respectively, an increased number of parts and an increased mounting area are required. This causes a serial problem with a portable terminal to which a strong demand for minimization is directed. It is to be noted that, while the examples of division of a gain adjustment element into a plurality of elements described above with reference to FIGS. 1 and 2 are applied to a transmission system, such division can be applied quite similarly also to a reception system.
Further, use of a field effect transistor (FET) made of gallium arsenite (GaAs) as a switching element of a switching circuit which handles a high frequency such as a microwave is increasing. Particularly, from an anticipation for reduction in size, augmentation in performance and reduction in cost of a circuit by integration and so forth, a monolithic microwave integrated circuit (MMIC) switch is considered significant.
For switching ICs, different circuit constructions are adopted depending upon the performances, functions and so forth required for them. Generally, an equivalent circuit to switching FETs made of GaAs is simply represented as resistors Ron connected in series when it is on, but as capacitors Coff connected in series when it is off. As an example, the resistors Ron in an on-state are approximately 2 .OMEGA.mm, and the capacitors Coff in an off-state are approximately 300 fF/mm.
Recently, personal communication using a portable telephone terminal or the like has spread widely. For such personal communication, communication bands lower than 2 GHz are used in almost all cases. Where a comparatively high isolation is required in such a frequency band as just mentioned and when it is necessary to establish 50 .OMEGA. matching with a port in an off-stage, a switching circuit having such a circuit construction as shown, for example, in FIG. 3 is used.
In particular, referring to FIG. 3, an FET Q11 and another FET Q12 are connected in series between a first input/output terminal 21 and a second input/output terminal 22. A shunt FET Q13 is connected between a common junction between the FETs Q11 and Q12 and the ground, and another shunt FET Q14 is connected in series to a resistor R19 between the second input/output terminal 22 and the ground. Resistors R11 to R14 are connected to the gates of the FETs Q11 to Q14, respectively.
Similarly, an FET Q15 and another FET Q16 are connected in series between the first input/output terminal 21 and a third input/output terminal 23. A shunt FET Q17 is connected between a common junction between the FETs Q15 and Q16 and the ground, and another shunt FET Q18 is connected in series to a resistor R20 between the third input/output terminal 23 and the ground. Resistors R15 to R18 are connected to the gates of the FETs Q15 to Q18, respectively.
In order to create a conductive path between the first input/output terminal 21 and the second input/output terminal 22 in the circuit having such a construction as described above, the FETs Q11 and Q12 and the shunt FETs Q17 and Q18 are put into an on-state while the shunt FETs Q13 and Q14 and the FETs Q15 and Q16 are put into an off-state. Since the FETs Q11 and Q12 are in an on-state, no loss is exhibited by this route. Meanwhile, since the shunt FETs Q13 and Q14 are in an off-state, a small signal leaks from this route to the ground. Accordingly, the route between the first input/output terminal 21 and the second input/output terminal 22 is put into a conducting state.
On the other hand, in a path between the first input/output terminal 21 and the third input/output terminal 23, since the FETs Q15 and Q16 are in an off-state, this path is in a non-conducting state. However, as the signal frequency increases, a signal leaks through off capacitances of the FETs Q15 and Q16 and the isolation characteristic is deteriorated. Therefore, the shunt FET Q17 is provided so that, as the shunt FET Q17 is put into an on-state, a signal leaking to the series FET Q15 is pulled in to the ground, and consequently, a high isolation can be assured. Further, the shunt FET Q18 in an on-state pulls in a signal leaking from the series FET Q16 to the ground and thus augments the isolation.
Meanwhile, the impedance as viewed from the third input/output terminal 23 appears to be 50 .OMEGA. since approximately only the resistor R20 (=50 .OMEGA.) appears because the impedance on the inner side of the IC than the FET Q16 does not appear due to the presence of the FET Q16, which is in an off-state, and if the impedance of the transmission line is 50 .OMEGA., then the impedances match each other. As described above, 50 .OMEGA. matching of an off port and a high isolation can be realized by the circuit of FIG. 3. The isolation characteristic of the circuit of FIG. 3 is illustrated in FIG. 4. As apparently seen from FIG. 4, the isolation at 2 GHz is as high as 66 dB.
In the circuit examples described above, it is assumed that mounting of the FET switches is ideal. Actually, however, various parasitic components are involved and cannot be ignored. For example, in a portable telephone terminal or the like, as the price decreases, a reduction of the cost for an IC used is also demanded. Therefore, since a ceramic package or the like, which has superior high frequency characteristics does not satisfy such a demand for reduction in cost, a plastic mold package is used in most cases. Where a plastic mold package is used, parasitic components which particularly have an influence on characteristics of a switch are inductance components which exist in series between signal terminals or a ground terminal of an IC and the outside of the IC. Such inductance components arise from wires for connection between the IC chip and I/O pins of the package, pins of the package and so forth. For example, an inductance of 1 nH or more is provided by one wire.
FIG. 5 is a circuit diagram showing a circuit wherein a common ground is provided on a chip. Referring to FIG. 5, like elements to those of FIG. 3 are denoted by like reference symbols. Where a common ground is used on a chip in this manner, a parasitic inductance Lb is provided between the common ground on the chip and the ground outside the IC. An isolation characteristic in this instance is illustrated in FIG. 6. From FIG. 6, it can be seen that the isolation is deteriorated significantly by a small parasitic inductance. For example, the isolation is deteriorated to 33 dB by the parasitic inductance Lb of 0.5 nH.
The reason is that, since the parasitic inductance Lb is provided between the common ground on the chip and the ground outside the IC, the common ground on the chip does not sufficiently act as the ground. As an example, a case wherein the route between the first input/output terminal 21 and the second input/output terminal 22 is rendered conducting is described. In this instance, a signal leaks to the common ground on the chip from the shunt FETs Q13 and Q14 on the on-state side and also from the shunt FET Q17 on the off-state side. Since the common ground on the chip does not sufficiently act as the ground, the leaking signal leaks to the third input/output terminal 23 through the shunt FET Q18 in an on-state thereby to deteriorate the isolation. In this manner, it is difficult to obtain a high isolation with a circuit on which a common ground is provided on a chip.
A circuit having such a circuit construction as shown in FIG. 7 has been proposed to prevent such deterioration in isolation as described above. Referring FIG. 7, the circuit shown is constructed such that the ground sides of shunt FETs Q13, Q14, Q17 and Q18 are directly connected to an ideal ground through a parasitic inductance Lb. Actually, the circuit is constructed such that the ground sides of the shunt FETs Q13, Q14, Q17 and Q18 on an IC are directly connected to I/O pins of the IC by wires. As apparently seen from FIG. 8, the isolation characteristic in this instance is improved very much.
As an example, where the parasitic inductance Lb is 0.5 nH, an isolation of approximately 60 dB is obtained. In this instance, however, since a number of I/O pins equal to the number of shunt FETs are required, the number of pins of the package increases, resulting in increase in size of the package. This is inconvenient for a device for which minimization is required such as a portable terminal.
Also another technique wherein grounds on an IC are formed independently of each other and individually connected to a die pad which serves as the ground of the IC by wires is available. This technique can improve the isolation and decrease the number of I/O pins of an IC package. However, as can be seen from FIG. 6, the isolation is deteriorated significantly by a little parasitic inductance Lb, and presence of a parasitic inductance of some magnitude between the die pad and an ideal ground outside the IC cannot be avoided although it is not so high as that provided by a wire. As a result, a very high isolation characteristic cannot be obtained.
If a switching circuit which is used in a quasi microwave band is constructed such that grounds on the chip are formed common as seen in FIG. 5, it is difficult to obtain a high isolation. In this instance, if the number of wires for connection between the common ground and a die pad of the packet is increased, then the isolation can be improved. However, as can be seen from FIG. 8, a very high isolation cannot be obtained.
Further, if the number of wires is increased, then although the inductances of the wires decrease and the isolation characteristic is improved considerably, since the number of pads for the wires on the chip increases, the chip size increases, resulting in an increase in cost. Further, where the circuit construction of FIG. 7 is employed, since the number of pins of the package increases, an increase in the package size also occurs. This is particularly inconvenient for such a device for which minimization is required such as a portable terminal. As described above, it is difficult to realize achievement of a high isolation, reduction in cost and minimization of a device in a quasi microwave band.